The most important weather model runs, based on global weather balloon (“radiosonde”) upper air measurements, are done twice a day— at 7 PM EST (00 UTC, also called 00z) and 7 AM EST (12 UTC, also called 12z).
These new upper air measurements often account for any significant change in the model forecasts. The latest NAM-NEST has a decrease in snow totals due to significantly more sleet.
Here’s the latest 00z model data, based on these new upper air measurements.
The 00z HRRR (High Resolution Rapid Refresh) shows snow developing between 12 noon and 2 PM Wednesday and ending 7-10 AM Thursday. It will become very WINDY Wednesday night.
The HRRR continues the trend for sleet to mix in with the snow in a large portion of the area during Wednesday evening, and then change back to snow before ending.
Despite the changeover to sleet at times, the latest HRRR has increased snow totals by Thursday morning—
The 00z NBM (Model Blend) shows a similar snow total forecast—
Several models are showing wind gusts in excess of 50 mph Wednesday evening.
Speaking of model forecast changes, the NAM NEST just available has more sleet and much less snow—
I think the NAM-NEST may be too warm with too much sleet. We’ll have to see the trends into tomorrow morning.
Right now I’m sticking with the HRRR and NBM snow forecasts above.
Here, just northwest of the city limits, the precipitation has changed to rain with a pause in the action. A lot has to happen to get to snow totals that the models have forecast! Perhaps things have been a bit over-forecast?
This morning’s GFS model continues with the same scenario all the models have been forecasting; heavy precipitation with dynamic cooling to allow temperatures to drop and accumulations to occur.
HOWEVER, the GFS Kuchera snow algorithm makes this storm a bust—
As mentioned last night, I’m not big on the Kuchera snow algorithm;it tends to seriously underestimate snowfall in past storms. (Last Friday’s storm had the Kuchera algorithm forecasting NO snow!)
Radar trends show a significant increase in precipitation rates in NJ, rotating westward into Pennsylvania. Despite all forecast parameters as snow, things have to change soon for this forecast to not be a bust. And as I type this, precipitation has changed back to snow! So let’s see what happens.
Here’s the very latest weather model data about the Philly snowstorm. The morning’s NAM data just became available. It shows a QPF of 1.35 inches water falling as snow between 7AM and 7PM.
Here’s the latest NAM 10:1 snow totals map. (Important- this map shows additional snow totals that need to be added to any snow that has already accumulated by 7 AM this morning.)
Temperatures are expected to fall to 32-33 as precipitation rates increase during the late morning and early afternoon. Expect heavy snow at times. Temperatures may rise at the end of the storm to 34!
In my neck of the woods, we’re already seeing the difficulties of predicting snow totals for a March snowstorm. Some surfaces and sidewalks have little accumulation, while our wood deck has about 2 inches right now.
[su_note note_color=”#ebf2d9″]With these issues with March snowstorms, we’re always predicting potential accumulations. Melting due to warm ground surfaces, radiant heat from solar insolation through clouds and snow compaction due to wetness can greatly affect final totals. Much depends on dynamic cooling to bring surface temperatures down.
Put another way, the precipitation rate is going to need to pick up considerably soon to meet the QPF values and the snow totals predicted by the NAM. [/su_note]